Systems for modulating a light source such as a laser beam and scanning that modulated light through an optical path to a light sensitive surface are well-known.
Such a system is shown in U.S. Pat. No. 3,984,171 of Hotchkiss. The Hotchkiss patent shows a video signal modulating a laser output. The modulator is placed in the light path of the laser and may switch the laser beam electro-optically or acoustic-optically in response to digitized video information. A beam expander is placed in the light path to expand the diameter of the beam so it may be refocused to a spot size required by the resolution of the system. Additional typical components used within the light path are fold mirrors to change the direction of the light path and a spherical compensator to provide the desired straight line or linear continuous scan on the recording medium from the curvilinear scan.
In such known systems, a scanning mirror is mounted on a motor, such as a galvanometer motor, to drive the mirror and the reflected light beam through an arc.
In such a scanning arrangement, it is desirable to maintain the optical path length as short as possible. The shortened optical path provides a diminished sensitivity to changes in the location of the various components of the optical path due to changes in temperature and other causes, as well as diminishing the sensitivity of any adjustments which may be required by the components and their locations. Additionally, a shortened optical path also enables a smaller, more compact unit to be built, with attendant savings in weight and space.
Additionally, a shortened light path requires less sensitive adjustment to convert the curvilinear scan of an oscillating mirror to a distortionless linear scan. A shortened light path also reduces light losses and scattering within the system which may reduce the quality of the system output, and minimizes energy required by the system.
The prior art arrangement of elements within an optical light path has some of the elements arranged independently of all of the other elements, requiring individual adjustment of these independently arranged elements relative to each other. This required a multiple step assembly operation where each of the elements had to be mounted, and then critically aligned with each other so that the optical path was within desired tolerances. In other cases where some of the elements were aligned with each other by an assembly having a common reference plane, the optical path including the photosensitive material transport was assembled separately from the optical assembly, and required individual adjustment to minimize any out of tolerance condition leading to distortion of the scan at the light sensitive surface.
This invention relates to the field of matrix printers and display devices wherein characters such as alphanumeric characters are generated by controlling a spot forming device that traverses or scans a copy area. More particularly, this invention relates to a mechanism for controlling the spot forming device as it traverses or scans a scanning path in a raster pattern that covers the area of a page of text. An application of this invention may be in a printing system such as an electrophotographic typesetting system wherein type characters are generated by a modulated light spot that scans a fixed linear path on the surface of an electrophotographic material, to selectively discharge areas of the page. This process places on the photoconductive surface an electrostatic image of text symbols which are developed and transferred to form a final copy by techniques somewhat similar to those currently employed in known electrostatagraphic copy machines. The preferred embodiment of such a machine is disclosed in copending application U.S. Ser. No. 037,698 filed May 10, 1979 and assigned to the common assignee.
The modulated light spot, and the raster scan, are controlled by a digital processor, which turns the spot forming device on and off in response to character contour data in normalized coordinates in a first digital memory, and input data on the desired locations on a page of the selected characters in a second digital memory.
A laser beam is used for the light source which is driven through a scanning arc by a scanning means such as an oscillating mirror. A light source such as a laser may be used in this device, wherein characters are electronically generated by repeatedly and rapidly scanning a beam of laser light across the image receiving medium. During scanning, the light beam is electronically switched on and off thereby forming a raster line which extends either vertically perpendicular to the line of characters or horizontally parallel to the lines of characters. The switching device is preferably a conventional acoustical optical modulator. Light from the modulator is reflected from a scanning mirror which creates a curvilinear scanned beam.
The output of the laser is a bundle of light rays which may be either larger or smaller than the resolution required by the system. The light rays must be focused to a spot compatible with the system resolution.
In accordance with the invention, a beam expander is placed in the optical path between the modulator and the scanning means. The beam expander, as in a Kepplerian telescope or a compound microscope, focuses the entering light rays on a first real image area smaller than the area of the ray bundle entering the beam expander. Then, in accordance with known optical techniques, the image at this first real focused area is expanded through the beam expander optics and then refocused on the photosensitive material. In accordance with known optical principles, the resolution, or the minimum spot size on the photosensitive surface, is a function of the size of the real imaged area produced in the beam expander, and is directly proportional to the size of the real focused spot produced within the beam expander. To obtain a sufficiently small spot in the system focal plane at the photosensitive image surface, a smaller spot size is required at the above described first real focused area within the beam expander.
In a preferred embodiment of the invention, an acoustical optical modulator produces light along two principle axes, a zero order axis and a first order axis. The first order axis corresponds to the energized or switched state of the modulator. An apertured stop is provided on the principle first order axis. The light of the first order then passes through the aperture, while zero order light is blocked.
The use of this apertured stop within the beam expander is particularly advantageous with this system where the modulator is placed between the light source and the beam expander.
In this system, a beam expander is used to focus the light from the laser to the aforesaid first real image spot. This focused spot is then reimaged through the beam expander and refocused onto the imaging surface to a cross-sectional area consistent with the resolution of the system.
The function of the beam expander is combined with the function of the modulator and with the aperture stop whereby the modulated light is imaged within the beam expander at its internal focal plane, and at a distance where the normally overlapping beams from the modulator have been separated by the optics of the beam expander. The beam expander optics separate the overlapping beams, and the aperture stop blocks the undesired beam, at a distance from the modulator where the angularly separated beams otherwise would be overlapping each other. The beam expander optics focus one of the beams on a blocking portion of the light stop within the beam expander.
To summarize, the apertured stop is placed within the beam expander at a focal plane of the beam expander where each bundle of light rays is imaged to a small focused area and whereby overlapping between the first and zero order light beams is eliminated.
In this way, the use of the apertured stop within the beam expander permits the beams of the zero and first order to be separated at a distance from the modulator output, where they would otherwise be overlapping, and permits the location of the beam expander closer to the output of the modulator. In this way, the total light path is shortened, providing the advantage of minimizing the adjustments required to bring the light path within the angular tolerances and reducing the amount of adjustment required to minimize distortion within the light path.
An additional feature of the invention is the use of an optical assembly which provides a plurality of mounting surfaces, referenced to a reference plane. According to the invention, portions of the light path elements, including the scanning mirror, the spherical correcting mirror and any fold mirrors are mounted on a common optical assembly, eliminating the need for any adjustments after the mounting step is completed. Additionally, and according to the principles of this invention, the photosensitive material transport is also mounted on the optical assembly and on its respective surface referenced to a common reference plane so that the optical path from the scanning mirror to the imaging surface on the photosensitive material can be aligned merely be assemblying the components. Any need for further alignment of the imaging surface and transport is eliminated.
Additionally, the shortened optical path also provides the capability of minimizing the size of the package, minimizing space and material requirements.
The laser and modulator may be mounted on a reference machined surface, or, according to the preferred embodiment may be connected together and mounted within diametrically-opposed screw pairs, so both may be adjusted simultaneously relative to the optical path defined by the machined surfaces on the optical assembly.
The scanning means is a scanning mirror which drives the light beam in a curvilinear path. It is mounted against the shaft of a galvanometer motor, or any other suitable driving device, by means such as a spring clip. This spring clip draws the backside surface of the mirror against the shaft, assuring that the surface of the mirror is in contact with the surface of the shaft and is aligned with the axis of the shaft.
In general, the system incorporates the features described above in the cross-referenced phototypesetting system applications characterized by the use of a microprocessor driven CRT display for typesetting, composition support, digital font storage, and laser raster recording.
The system employs electrostatographic printing and produces a rapid and high quality output copy suitable for typesetting.
In a typesetting system, the quality of the copy is compared to that produced by metal type. Producing such quality presents difficulties especially where normalized encoded character contour digital data may be greatly "magnified" in forming larger characters. Maintaining suitable quality requires the features disclosed below, in precise alignment in a minimum space, to produce a substantially distortionless scan. The features disclosed and claimed contribute to a high print quality. The novel arrangement of the beam expander reduces the length of the optical path, reducing the opportunity for irregularities in the beam, and the alignment means provide more precise alignment of the beam with the imaging surface. These combine to reduce misalignment and degradation of print quality due to vibration, temperature differences, and random unavoidable minor irregularities in optical components, providing print quality equal to that of metal type in a production environment.
Additionally disclosed in the system is an arrangement of the optical path defining the position of the scanning mirror axis of rotation, the center of curvature of the spherical compensating reflector, and the system focal plane. In particular, the straight line scan locus is produced at a focal plane perpendicular to the axis of rotation of the scanning device, as well as passing through the center of curvature of the spherical compensating off-axis reflector as explained below.
This relationship is not disclosed in the prior art wherein the combination of the off-axis compensating spherical mirror and a light source scanned through an arc, produces the desired straight line locus in an off-axis system, substantially free from distortion.